NMAC AS THE ACFT WERE CROSSING OVER THE BOUNDARY OF THE FIELD.

Narrative

A NEAR MISS TOOK PLACE IN WHICH CARGO ACFT CLRED TO LAND SJC RWY 30R WAS FORCED INTO GOING AROUND WHEN IT BECAME EVIDENT THAT ANOTHER ACFT (SMA BASED IN SJ) WAS ALSO LINING UP TO LAND ON SJC'S 30R. BOTH ACFT WERE APCHING THE THRESHOLD OF RWY 30R AT ABOUT 50'. CARGO ACFT WAS ON A STRAIGHT IN FROM APPROX 15 MI OUT. SMA WAS TURNING A CLOSE IN LEFT BASE TO FINAL. UPON SIGHTING EACH OTHER; EACH ACFT TURNED QUICKLY AWAY FROM ONE ANOTHER TOWARDS THE INSIDES OF THEIR RESPECTIVE PATTERNS. CARGO ACFT MADE A SHARP RIGHT TURN; SMA MADE A SHARP LEFT TURN. IN LATER SPEAKING WITH SJ'S CHIEF CTLR; IT IS APPARENT THAT THERE WAS A COM BREAKDOWN BTWN THE CTLR OF THE SMA AND THE SMA PLT. BUT; BECAUSE THE CTL TWR FREQS ARE SEPARATE BTWN THE LEFT AND RIGHT RWYS; I HAD NO INDICATION OF THE ENSUING INCIDENT. DISTANCE BTWN ACFT; JUDGED BOTH BY MYSELF AND 3 WITNESSES ON THE GND; WAS ESTIMATED AT 10'--UNCOMFORTABLY CLOSE! BECAUSE OF THE SEVERITY OF THIS SITUATION; I PLAN TO FILE A FORMAL NEAR MISS RPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR REQUESTED TO HEAR TAPES BUT HAS NOT HAD A CHANCE TO LISTEN. SMA X HAD REQUESTED CONFIRMATION OF WHICH RWY HE WAS TO LAND ON BUT THE TWR FAILED TO RESPOND. ACFT HAD BEEN MAKING CLOSE IN PATTERNS FOR RWY 29. RPTR FILED AN NMAC RPT WITH THE FAA AND HAS HEARD NOTHING FURTHER. HE WAS MAKING A CLOSE IN APCH SINCE HIS OPERATION HAS HIM IN AND OUT 3 OR 4 TIMES/DAY.